1. Field of the Invention
The present invention relates to a switching regulator for controlling transmit antennas of variable impedance, particularly transmit antennas of a keyless access authorization system in or for a vehicle, a transceiver circuit having a switching regulator of this type, and a keyless access control system in or for a vehicle.
2. Description of the Background Art
The invention and its underlying problem are described hereafter with reference to a keyless access control system in a motor vehicle, however, without limiting the invention thereto. It is pointed out in particular that the invention may certainly also be used in other applications, for example, in switching power supplies.
Modern electronic anti-theft systems in a motor vehicle comprise an access authorization system, for example, a central locking device that can be activated by a mechanical or electronic key. Modern access authorization systems in motor vehicles enable in addition or alternatively keyless access as well. This type of system is, for example, the keyless access system with the name “Adrima” that was developed and sold by the company ATMEL Germany GmbH and works with a so-called passive access control system (“passive access”). In the passive access control system, a question-answer dialog is initiated when a user approaches his motor vehicle. In this case, a request signal is transmitted by a transmitter with a transmit antenna in the vehicle to a transponder carried by the user. When said transponder receives the request signal, it transmits, e.g., a cryptologically encoded response signal. In the motor vehicle, this response signal is received by means of a receive antenna, decoded, and compared with an expected set signal. If the response signal and set signal match, i.e., in the case of a successful authentication, the doors and optionally also the anti-theft device are unlocked.
The German patent DE 195 46 171 C1, which corresponds to U.S. Pat. No. 5,838,074, describes this type of anti-theft system for use in a motor vehicle, in which bidirectional data communication is established by means of magnetic coupling between a vehicle-side transceiver and a mobile transponder. The data communication occurs by turning on and off an alternating field generated magnetically by the transceiver according to the data to be transmitted. For this purpose, the transceiver has an inductive antenna resonant circuit, which is excited by a drive circuit. The inductor of the transceiver antenna resonant circuit is coupled magnetically to a corresponding inductor of the transponder antenna resonant circuit.
A system for passive access control may have, e.g., a control device in the vehicle and typically several inductive antennas, which are distributed in the vehicle and are connected to the control device via connecting lines generally of different lengths. These inductive antennas are activated by a switching regulator within the control device. It is problematic here that as the length of the connecting lines increases, their parasitic capacitances and inductances also increase, so that these are no longer negligible. In addition to this self-capacitance and self-inductance of the connecting lines, a capacitive fraction is also to be considered that results from the type and installation of the connecting lines in the vehicle, for example, from the effect of the vehicle body.
Another problem results from the type of transmit antennas, which ideally are to be made as identical as possible, but which in the specific technical implementation, e.g., due to the employed production technology, in fact have a more or less broad variation in the antenna impedance.
The control of the plurality of antennas is problematic here. Typically, the control device has a switching regulator for controlling all transmit antennas within the motor vehicle. The switching regulator and the output driver connected to the switching regulator are designed to control all transmit antennas of a motor vehicle with a predefined signal level that is as identical as possible, whereby a minor voltage or current fluctuation can be compensated by the switching regulator. Because of the at times highly varying antenna impedances, however, the current flowing through the different antennas also fluctuates and thereby accordingly greatly also the dropping voltage at the transmit antenna. Particularly in the vehicle sector, the antenna impedance can vary by up to several 10 ohms, so that the antenna current to be driven by the control amplifier through the antennas can vary as a result by up to several amperes. This means that the switching regulator must be designed to ensure a control stability over a relatively large load range. The switching regulator must therefore be designed to be able to switch output voltages in the range from a few volts to several 10 volts.
The particular problem here is that the switching regulator can thereby become unstable in its control stability, which overall has the result that a functioning control is no longer assured. There is the need therefore to provide a switching regulator that can regulate as precisely as possible the output voltage provided for the antenna and thereby the antenna current also during highly varying input voltages.
U.S. Pat. No. 5,903,452 describes a switching regulator with a control circuit that provides a compensation signal for regulation. The voltage ramp of the compensation signal in this case depends on a coil current tapped directly at the antenna coil and fed back, so that the regulation here is based directly on the coil current.